In the field of wireless communications using radio waves, a technique called Frequency Division Duplex (FDD) in which one frequency band is used to transmit and another is used to receive a transmission is used in communication schemes such as W-CDMA. If a station uses one antenna to perform the bidirectional communication, the station uses a duplexer to prevent a signal transmitted from the station from entering a system that directly receives signals from other stations. The frequency characteristics of the duplexer is typically invariant. Therefore, for example in a communication device that uses multiple frequencies or bandwidths, duplexers for the respective frequencies or bandwidths are provided and a switch is used to make switching between them (Masaaki Koiwa, Fumiyoshi Inoue, and Takashi Okada, “Multiband Mobile Terminals”, NTT DoCoMo Technical Journal Vol. 14, No. 2, pp. 31-37, July, 2006).
There is another technique called Time Division Duplex (TDD) in which transmission and reception are performed with the same frequency in different timings. In the TDD, sharing of an antenna is generally accomplished by using a switch. Global System for Mobile Communications (GSM), which is not a TDD system, makes switching between transmission and reception using a switch, like the TDD. Accordingly, terminals supporting the GSM use a switch to use an antenna for both transmission and reception. Mobile terminals that support both W-CDMA, which uses FDD, and GSM include transmission/reception systems for both schemes and use a switch to make switching between the systems (Takashi Okada, “Mobile Terminal RF Circuit Technology for Increasing Capacity/Coverage and International Roaming”, NTT DoCoMo Technical Journal Vol. 16, No. 2, pp. 45-53, July, 2008). Since TDD uses the same frequency for transmission and reception, the duplexer described above cannot be provided in transmission and reception paths. Therefore, a communication device that supports both FDD and TDD requires transmission/reception systems for FDD and transmission/reception systems for TDD.
However, these methods have a problem that the circuit area and the number of components increase as the number of frequencies or bandwidths used increases. Furthermore, the need for transmission and reception circuits for both FDD and TDD also leads to increase in the circuit area and the number of components.
In view of these circumstances, an object of the present invention is to provide a circuit that function as both of a duplexer whose frequency band and/or center frequency can be changed and a switch for TDD, that is, to provide a multimode frontend circuit.